by
and
The images shown for Figures 2-8 are available for downloading
as tiff files for viewing in NIH Image or other program. Download
links are on the pages for the respective figures
Figure 1. Map of the Adirondack mountains, NY, showing location
of samples examined in this study. Light gray shading delineates
the outcrop exposure of the anorthosite (after McLelland and Chiarenzelli,
1990); surrounding rocks are ortho- and paragneisses of diverse
affinities; darker gray is water. Contours show isotherms of temperature
calculated from feldspar and oxide thermometry by Bohlen et al.
(1980b, 1985).
Figure 9. AFM and CFM diagrams showing compositions of pyroxenes
and garnets from samples ET-10 and SAR-1. Tie lines have been
drawn between pyroxene rim and garnet rim compositions.
Figure 10. Pressure-temperature diagrams showing contours for
Fe/(Fe+Mg), Al and Na in clinopyroxene (a) and Fe/(Fe+Mg) and
Al in orthopyroxene (b) for the assemblage clinopyroxene + orthopyroxene
+ plagioclase + quartz. Arrow depicts a P-T path that is consistent
with the observed zoning in pyroxene.
Figure 11. Pressure-temperature diagrams showing contours for
Fe/(Fe+Mg), XGrs and molar abundance in garnet (a), (b) and (c);
Fe/(Fe+Mg) and Al in clinopyroxene and orthopyroxene (d), (e)
and (g), (h), respectively; and XAn in plagioclase (f) for the
assemblage garnet + clinopyroxene + orthopyroxene + plagioclase
+ quartz. The arrow shows a P-T vector that is consistent with
the observed zoning in all minerals.
Figure 12. (a) Plot of volume per cent of minerals versus temperature
along a cooling path of 6 bars/°C. Garnet, quartz and clinopyroxene
are produced at the expense of plagioclase and orthopyroxene.
The measured modal amount of garnet (approximately 5 volume per
cent) requires a temperature decrease of approximately 60 °C.
(b) Schematic AFM diagram showing the shift in garnet and pyroxene
compositions along a cooling path of 6 bars/°C.
Figure 13. Plots of the pyroxene quadrilateral showing the integrated
compositions of pyroxene megacrysts from three samples (AS-31,
ET-10, SL-8). The simple normalization was used for these plots,
so the temperatures of metamorphic pyroxenes are believed to be
underestimated in these diagrams by 20-70 °C.
Figure 14. Plots of the pyroxene quadrilateral showing the compositions
of coexisting metamorphic orthopyroxene + clinopyroxene from all
samples. Solid symbols are core compositions, open symbols rims
(cpx only). (a) Simple normalization assuming all Fe is Fe2+.
(b) Simple normalization assuming Fe3+ calculated from stoichiometric
constraints. (c) Compositions of coexisting pyroxene cores plotted
using the thermodynamic projection scheme described in Appendix
A assuming Fe3+ calculated from stoichiometric constraints.
Figure 15. Plot of calculated temperatures using Fe-Mg partitioning
between garnet + pyroxene rims. Black rectangles: all Fe is Fe2+;
Gray rectangles: Fe3+ calculated from stoichiometric constraints.
Length of rectangles reflects error associated with analytical
uncertainty. Numbers and horizontal lines are average temperatures.
(a) Garnet + clinopyroxene rim calculated using calibration of
Powell (1985). (b) Garnet + orthopyroxene rim calculated using
calibration of Harley (1984a). Rectangle labeled "core"
for sample AS-19E is temperature calculated using orthopyroxene
core.
Figure 16. (a) Plot of calculated temperatures using Fe-Mg partitioning
between garnet + hornblende (gray bars; calibration of Graham
& Powell, 1984) and garnet + biotite (black bars; Calibration
of Patiño-Douce, et al., 1993). Average garnet + hornblende
temperature, excluding sample ET-10 is 723 °C. Average garnet-biotite
temperature, excluding samples SL-8 and SR-18 is 817 °C. Length
of boxes reflects error associated with analytical uncertainty
(± 25 °C). (b) Plot of calculated temperature versus
F content for samples shown in (a). The effect of F is to lower
the calculated temperature substantially.
Figure 17. P-T diagram showing the results of garnet + plagioclase
barometry. Solid lines are from garnet + plagioclase + clinopyroxene
+ quartz barometry, dashed lines are from garnet + plagioclase
+ orthopyroxene + quartz barometry (calibrations of Eckert et
al., 1991), dotted lines are from garnet + plagioclase + hornblende
+ quartz barometry (calibration of Kohn & Spear, 1990). Gray
box shows peak P-T conditions for central Adirondacks from Bohlen
et al. (1985).
Figure 18. P-T diagram summarizing the results of this and other
studies on the P-T evolution of the Adirondack highlands with
the geochronologic constraints summarized by McLelland & Chiarenzelli
(1990).
Figure A1. (a) AC(FM) diagram showing the graphical basis of the
"thermodynamic projection" scheme used to project from
non-quadrilateral components onto the pyroxene quadrilateral.
Large dots are a representative opx + cpx assemblage. Schematic
tie lines are shown to define the two-phase opx + cpx region.
Points "c" are the intersection of the two-phase region
with the Al-free diopside - enstatite join. (b) AC(FM) diagram
showing the effect of different projection schemes on plotting
positions of pyroxenes on the pyroxene quadrilateral (diopside
- enstatite join). The original ortho and clinopyroxenes contain
0.08 and 0.14 cations/six oxygens of Al + Ti + Fe3+, respectively.
Projection point "a" is from the simple normalization
scheme. Projection point "b" projects along exchange
vectors. Projection point "c" results from a thermodynamic
projection (Spear, 1988). Temperatures shown reflect isotherms
for coexisting ortho and clinopyroxene at Fe/(Fe+Mg) = 0.4 from
Sack & Ghiorso (1994). Note that the temperatures inferred
from the three projection schemes each differ by 125 °C. Di
= diopside, En = enstatite, Cats = Ca-tschermaks, Mg-Tsch = Mg-tschermaks.